public class L617 {

    class Solution {
        public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
            if (root1 == null && root2 == null) {
                return null;
            }
            if (root1 == null) {
                return root2;
            }
            if (root2 == null) {
                return root1;
            }
            root1.left = mergeTrees(root1.left, root2.left);
            root1.right = mergeTrees(root1.right, root2.right);

            root1.val += root2.val;
            return root1;
        }
    }
}
